1. Field of the Invention
The present invention relates to a power supply for a motor, and more particularly, to a power supply for a motor and a method of supplying power to the motor, which improve energy efficiency.
2. Description of the Related Art
A three-phase motor is the kind of motor in which coils are spaced about a rotor of the motor and are connected to operate from a three phase power source . As shown in FIG. 6, a motor power supply 100 for the three-phase motor is employed to output a three-phase voltage to drive a motor 117, and comprises an alternating current (AC) power supply input part 101 which receives a commercial AC power, such as for example, 110/220V, a soft charging circuit 103 which prevents an excessive inrush current from flowing when power is initially supplied, a direct current (DC)-conversion circuit 106 which converts AC power into DC power, an overvoltage-protection circuit 112 which protects the DC-conversion circuit 106 from overvoltage, and an inverter 116 which converts the DC power into three-phase AC power having various frequencies. The inverter 116 comprises a pulse width modulation (PWM) part (not shown) which generates a PWM signal, and a plurality of transistors which are switched on/off according to a square waveform signal output from the PWM part. Further, the motor power supply 100 comprises a micom (not shown) which controls an output of the inverter 116 to be on/off by switching on/off the transistors according to the PWM control signal, and controls a rotation speed of the motor 117 by changing a frequency of the output of the inverter 116.
The motor power supply 100 employs a rectifying circuit 105 and a capacitor circuit 107 which function as a voltage doubler, when a voltage of 220 Vrms is needed and a voltage of 100˜110 Vrms is input. In order to smoothly control the motor 117, the DC-conversion capacitor circuit 107 should have sufficiently large capacity. Therefore, when power is initially supplied, the capacitors CDC1 and CDC2 of the DC-conversion capacitor circuit 107 are charged, causing very high inrush current if the current is not otherwise limited. The high inrush current is likely to generate a current spike, thereby overloading the AC power input and tripping a circuit breaker to break the power, affecting other devices connected to the AC power input part, or shortening a life span of the capacitors of the DC-conversion capacitor circuit 107.
The soft charging circuit 103 limits the inrush current. As shown in FIG. 6, when power is initially supplied, a relay 102 is in the state of being switched off, so that the capacitors CDC1 and CDC2 are charged with a positive voltage passing through an inrush current limiting resistor 104 and a diode D1 and a negative voltage passing through the inrush current limiting resistor 104 and a diode D2, respectively. A total voltage applied to the pair of capacitors CDC1 and CDC2 in the DC-conversion circuit will be referred to as VPN. When VPN is equal to a charging stop voltage V1, the relay 102 is switched on, so that the power from AC power input part 101 is directly supplied to the diodes D1 and D2 in the rectifying circuit 105.
While the motor 117 operates, regenerative power may be generated by the inverter 116 and supplied to the DC-conversion circuit 106. When the capacitors CDC1 and CDC2 are over-charged with the regenerative power, the overvoltage-protection circuit 112 senses that VPN is higher than a predetermined voltage (i.e., a critical overvoltage), and switches on a transistor 109 of the overvoltage-protection circuit 112, to dissipate the regenerative power as heat energy through a resistor 110 of the overvoltage-protection circuit 112.
However, in the conventional motor power supply, the regenerative energy generated while the motor 117 operates is wasted as the heat energy by the resistor 110 of the overvoltage-protection circuit 112, thereby decreasing energy efficiency. Further, in the conventional motor power supply 100, there are needed the resistor 104 to limit the inrush current and the resistor 110 to protect against the overvoltage, so that a size of the motor power supply 100 is increased.